how to calculate sidereal time? if given GMT and lat/lan of place.?

copied from http://aa.usno.navy.mil/faq/docs/GAST.html :

Sidereal time is a system of timekeeping based on the rotation of the Earth with respect to the fixed stars in the sky. More specifically, it is the measure of the hour angle of the vernal equinox. If the hour angle is measured with respect to the true equinox, apparent sidereal time is being measured. If the hour angle is measured with respect to the mean equinox, mean sidereal time is being measured. When the measurements are made with respect to the meridian at Greenwich, the times are referred to as Greenwich mean sidereal time (GMST) and Greenwich apparent sidereal time (GAST).

Given below is a simple algorithm for computing apparent sidereal time to an accuracy of about 0.1 second, equivalent to about 1.5 arcseconds on the sky. The input time required by the algorithm is represented as a Julian date (Julian dates can be used to determine Universal Time.)

Let JD be the Julian date of the time of interest. Let JD0 be the Julian date of the previous midnight (0h) UT (the value of JD0 will end in .5 exactly), and let H be the hours of UT elapsed since that time. Thus we have JD = JD0 + H/24.

For both of these Julian dates, compute the number of days and fraction (+ or -) from 2000 January 1, 12h UT, Julian date 2451545.0:

D = JD - 2451545.0

D0 = JD0 - 2451545.0

Then the Greenwich mean sidereal time in hours is

GMST = 6.697374558 + 0.06570982441908 D0 + 1.00273790935 H + 0.000026 T2

where T = D/36525 is the number of centuries since the year 2000; thus the last term can be omitted in most applications. It will be necessary to reduce GMST to the range 0h to 24h. Setting H = 0 in the above formula yields the Greenwich mean sidereal time at 0h UT, which is tabulated in The Astronomical Almanac.

The following alternative formula can be used with a loss of precision of 0.1 second per century:

GMST = 18.697374558 + 24.06570982441908 D

where, as above, GMST must be reduced to the range 0h to 24h. The equations for GMST given above are adapted from those given in Appendix A of USNO Circular No. 163 (1981).

The Greenwich apparent sidereal time is obtained by adding a correction to the Greenwich mean sidereal time computed above. The correction term is called the nutation in right ascension or the equation of the equinoxes. Thus,

GAST = GMST + eqeq.

The equation of the equinoxes is given as eqeq = Δψ cos ε where Δψ, the nutation in longitude, is given in hours approximately by

Δψ ≈ -0.000319 sin Ω - 0.000024 sin 2L

with Ω, the Longitude of the ascending node of the Moon, given as

Ω = 125.04 - 0.052954 D,

and L, the Mean Longitude of the Sun, given as

L = 280.47 + 0.98565 D.

ε is the obliquity and is given as

ε = 23.4393 - 0.0000004 D.

The above expressions for Ω, L, and ε are all expressed in degrees.

The mean or apparent sidereal time locally is found by obtaining the local longitude in degrees, converting it to hours by dividing by 15, and then adding it to or subtracting it from the Greenwich time depending on whether the local position is east (add) or west (subtract) of Greenwich.

Sidereal Time Calculator

Local Sidereal Time

calculate sidereal time gmt latlan place

RE:

how to calculate sidereal time? if given GMT and lat/lan of place.?

any local time is wrt. GMT. which is tropical. sidereal time varies wrt. lattitude longitude of local places. please give specific formulae.

Look here for a quick explanation: http://tycho.usno.navy.mil/sidereal.html

You have to know how to do things I've never heard of.

Why do you need to know? I've been an astronomer for fifty years and never once in that time have I ever needed to know the sidereal time. If I did want to know it, I'd just look it up in my Starry Night software, and let it do the calculating for me.